TOKYO (AP) - A cow suspected of having mad cow disease has been found in northern Japan in what may be the nation's 20th case of the illness, an official said Saturday.

Preliminary tests on the animal at a slaughterhouse in the northern prefecture of Hokkaido were positive, and dairy officials sent samples to two local university laboratories for more precise tests, said prefectural official Hitoshi Watanabe.

Further details, such as the animal's age and whether it was a dairy or beef cow, were not immediately available, Watanabe said.

Final test results could be announced in the next several days.

Japan has found 19 animals infected with the fatal illness - formally known as bovine spongiform encephalopathy, or BSE - since the country's first case was detected in 2001. Tokyo has since checked every slaughtered cow before it enters the food supply.

Eating meat from a BSE-infected infected animal is thought to cause the fatal, brain-wasting variant Creutzfeldt-Jakob disease in humans.

Japan has banned American beef imports since December 2003, immediately after the U.S. first case of mad cow disease was confirmed in Washington state.

Under U.S. pressure, a Japanese government panel last month took a step toward partially lifting a ban on U.S. beef imports, but the decision still has to be approved by the government.

In February, Japan confirmed its first case of Creutzfeldt-Jakob disease following the death of a man with symptoms of the illness. Japanese health authorities have said they believed the man contracted the disease during a monthlong visit to Britain - where mad cow disease first surfaced - in 1989.